Issue when using for var x in foo

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle', 
  enumerableProperties: Ember.computed(function() {
    let list = [];
    for (var x in [1,2,3,4,5]) { 
      list.push(x);
    }

    return list;
  })
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
  <ul>
  {{#each enumerableProperties as |prop|}}
    <li>{{prop}}</li>
  {{/each}}
  </ul>
<br>
<br>

twiddle.json

{
  "version": "0.10.4",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.7.0",
    "ember-data": "2.7.0",
    "ember-template-compiler": "2.7.0"
  },
  "addons": {}
}